High-frequency semiconductor wafer processing apparatus and method

ABSTRACT

A plasma process apparatus capacitor operation significantly above 13.56 MHz can produce reduced self-bias voltage of the powered electrode to enable softer processes that do not damage thin layers that are increasingly becoming common in high speed and high density integrated circuits. A nonconventional match network is used to enable elimination of reflections at these higher frequencies. Automatic control of match network components enables the rf frequency to be adjusted to ignite the plasma and then to operate at a variable frequency selected to minimize process time without significant damage to the integrated circuit.

This is a continuation of application Ser. No. 07/774,127 filed Oct. 11,1991, now U.S. Pat. No. 5,223,457, which is a continuation ofapplication Ser. No. 07/416,750 filed Oct. 3, 1989, now abandoned.

In the figures, the first digit of a reference numeral will indicate thefirst figure in which is presented the element indicated by thatreference numeral.

BACKGROUND OF THE INVENTION

This invention relates in general to semiconductor wafer processingapparatus and relates more particularly to the use of a plasma apparatusfor wafer cleaning, wafer deposition and wafer etching. For example, theplasma etching of wafers is attractive because it can be anisotropic,can be chemically selective, can produce etch under conditions far fromthermodynamic equilibrium, utilizes a reduced amount of etchantchemicals compared to wet etch processes and produces a significantlyreduced amount of waste products. The reduction of etchant chemicals andwaste products produces a cost savings. The anisotropic etch enables theproduction of substantially vertical sidewalls which is important inpresent day processes in which the depth of etch and feature size andspacing are all comparable. The ability to choose etch chemicals andprocess parameters to produce chemical selectivity of the etch enablesthese choices to be made to etch the desired material withoutsubstantially etching other features of the integrated circuits beingproduced. Choices of process parameters that produce process conditionsfar from thermodynamic equilibrium can be used to lower processtemperature, thereby avoiding high temperatures that can deleteriouslyaffect the integrated circuits under fabrication.

In FIG. 1 is shown a plasma reactor 10. This reactor includes analuminum wall 11 that encloses a plasma reactor chamber 12. Wall 11 isgrounded and functions as one of the plasma electrodes. Gases aresupplied to chamber 12 from a gas source 13 and are exhausted by anexhaust system 14 that actively pumps gases out of the reactor tomaintain a low pressure suitable for a plasma process. An rf powersupply 15 provides power to a second (powered) electrode 16 to generatea plasma within chamber 12. Wafers 17 are transferred into and out ofreactor chamber 12 through a port such as slit valve 18.

A plasma consists of two qualitatively different regions: thesubstantially neutral, conductive plasma body 19 and a boundary layer110 called the plasma sheath. The plasma body consists of substantiallyequal densities of negative and positive charged particles as well asradicals and stable neutral particles. The plasma sheath is an electrondeficient, poorly conductive region in which the electric field strengthis large. The plasma sheath forms between the plasma body and anyinterface such as the walls and electrodes of the plasma reactor chamberand the rf electrodes.

Semiconductor process plasmas are produced by a radio frequency (rf)field at 13.56 MHz that couples energy into free electrons within thechamber, imparting sufficient energy to many of these electrons thations can be produced through collisions of these electrons with gasmolecules. Typically, the walls of the reactor chamber are metal (thoughoften coated with thin insulating layers) so that they can function asone of the rf electrodes. When the walls do not function as one of theelectrodes, they still affect the process by confining the plasma and bycontributing capacitive coupling to the plasma reactor.

The 13.56 MHz frequency is substantially universally utilized in plasmareactors because this frequency is an ISM (Industry, Scientific,Medical) standard frequency for which the government mandated radiationlimits are less stringent than at non-ISM frequencies, particularlythose within the communication bands. This substantial universal use of13.56 MHz is further encouraged by the large amount of equipmentavailable at that frequency because of this ISM standard. Other ISMstandard frequncies are at 27.12 and 40.68 MHz, which are first andsecond order harmonics of the 13.56 MHz ISM standard frequency. Afurther advantage of the 13.56 MHz frequency is that, since the lowesttwo order harmonics of this frequency are also ISM standard frequencies,equipment utilizing 13.56 MHz is less likely to exceed allowable limitsat harmonics of the fundamental frequency of such equipment.

When the powered rf electrode is capacitively coupled to the rf powersource, a dc self bias V_(dc) of this electrode results. The magnitudeof this self bias V_(dc) is a function of the ion density and electrontemperature within the plasma. A negative self-bias dc voltage V_(dc) ofthe powered electrode on the order of several hundreds of volts iscommonly produced (see, for example, J. Coburn and E. Kay, Positive-ionbombardment of substrates in rf diode glow discharge sputtering, J.Appl. Phys., 43, p. 4965 (1972). This self bias dc voltage V_(dc) isuseful in producing a high energy flux of positive ions against thepowered electrode. Therefore, in a plasma etching process, a wafer 17 tobe etched is positioned on or slightly above the powered electrode 16 sothat this flux of positive ions is incident substantially perpendicularto the top surface of the wafer, thereby producing substantiallyvertical etching of unprotected regions of the wafer.

These high voltages enable etch rates that are required for the etchprocess to be commercially attractive. Because of the susceptibility ofthe small (submicron) geometry devices available today to damage by asmall amount of particulates, integrated circuit (IC) process systemsare available that enable several IC process steps to be executed beforereexposing the wafer to ambient atmosphere (see, for example, themultichamber system illustrated in U.S. Pat. No. 4,785,962 entitledVacuum Chamber Slit Valve, issued to Masato Toshima on Nov. 22, 1988).This small geometry has also produced a trend toward single waferprocess steps (as opposed to multiwafer processing steps that are commonin larger geometry devices) so that processing can be sufficientlyuniform over the entire wafer that these small geometry features can beproduced throughout the wafer.

Because the wafer throughput of the system is limited to the throughputof the slowest of the series of process steps within such a system, itis important that none of these sequential steps take significantlylonger than the other steps in the process or else such slow step willserve as a bottleneck to system throughput. Presently, typical systemthroughput is on the order of 60 wafers per hour. For example, thefundamental etch prior to metal-2 deposition is performed at a rateequivalent to a 250 Angstroms per minute silicon dioxide etch rate. Thispermits the removal of approximately 70 Angstroms of aluminum oxide incontacts to aluminum metal-1 in approximately 40 seconds using anonselective argon-only process. These etch conditions are usedroutinely in wafer fabrication and produce a 1500-1600 volt self bias atthe powered electrode.

Transistor speed specifications and high device densities in the mostmodern MOS integrated circuits have required the use of shallowjunctions and thin gate oxides. Unfortunately, such IC structures aresensitive to ion bombardment by high energy ions such as those utilizedwith conventional 13.56 MHz plasma etch apparatus. Therefore, it isadvantageous in such IC processing to reduce the self-bias voltage ofthe powered electrode to less than 500 volts negative self-bias using anonselective argon-only process. Because wafer damage decreases withdecreasing self-bias voltage, it would be even more advantageous tooperate at self-bias voltages closer to 300 V. Unfortunately, at 13.56MHz, this reduction of self-bias results in a much slower etch rate,which thereby significantly degrades process throughput.

One solution has been to enhance the etch rate by use of magnets thatproduce containment fields that trap ions within the vicinity of thewafer, thereby increasing the ion density at the wafer. The magneticfield confines energetic ions and electrons by forcing them to spiralalong helical orbits about the magnetic field lines. Such increased iondensity at the wafer produces a concomitant increase in etch ratewithout increasing the self bias potential, thereby enabling throughputson the order of 60 wafers per hour without damaging the wafers. Ineffect, the etch rate is preserved by increasing the current level tocounter the decreased voltage drop across the plasma sheath at thewafer. Unfortunately, nonuniformities of the magnetic field of such"magnetically enhanced" plasma etching systems exhibit a decreaseduniformity of etch rate over the surface of the wafer.

To improve uniformity over the surface of the wafer, in one such system,the wafer is rotated about an axis that is perpendicular to and centeredover the surface of the powered electrode. This produces at the wafersurface a time-averaged field that has cylindrical symmetry and animproved uniformity over the wafer and therefore produces increased etchuniformity over the surface of the wafer. However, this rotationproduces within the plasma chamber undesirable mechanical motion thatcan produce particulates and increase contamination. Alternatively, arotating magnetic field can be produced by use of two magnetic coilsdriven by currents that are ninety degrees out of phase. Unfortunately,the controls and power supplies for this scheme are relatively expensiveand the etch uniformity is still not as good as in a plasma etchapparatus that does not include such magnets.

Another solution to enhance the rate of plasma processing of wafers isthe recently developed technique of electron cyclotron resonance. Thistechnique has application to wafer cleaning, etching and depositionprocesses. In this technique, a plasma is produced by use of a microwavesource and a magnetic containment structure. Unfortunately, thistechnique, when applied to etching or chemical vapor deposition,exhibits poor radial uniformity and low throughput. In addition, itrequires expensive hardware that includes: (1) a complex vacuum pumpingsystem; (2) a microwave power supply that must produce microwave powerat an extremely accurate frequency and power level; (3) a large magneticcontainment system that may include large electromagnets; and (4) an rfor dc power supply connected to the wafer electrode.

SUMMARY OF THE INVENTION

In a conventional plasma reactor, an igniter electrode produces, withina low pressure gas, high energy electrons that have enough energy toionize within the reactor chamber atoms and molecules struck by thesehigh energy electrons. This results in a cascade of electrons thatproduce a plasma consisting of electrons, ions, radicals and stableneutral particles. The plasma is then maintained by a powered electrodeof voltage lower than that of the igniter electrode. Sufficient rf poweris coupled into the plasma to maintain a desired ion concentration,typically on the order of 10⁸ -10¹¹ cm⁻³. Typical the frequency of therf power is in the range from 10 kHz to 30 MHz, but the most commonfrequency is 13.56 MHz because this is high enough to produce reasonableion concentrations and is an ISM (industry, scientific, medical)standard frequency that does not interfere with telecommunications.

Because the electrons are on the order of thousands to hundreds ofthousands of times lighter than the plasma ions, the electronsexperience a proportionately greater acceleration than the ions andtherefore acquire kinetic energies that are significantly greater thanthose acquired by the ions. The effect of this is that the energy fromthe rf field is strongly coupled into electron kinetic energy and isonly very weakly coupled into ion kinetic energy. These high energyelectrons are also referred to as high temperature electrons. As afurther result of this large mass difference between the electrons andthe ions, collisions between the high energy electrons and the ions doesnot transfer much of the electron energy to the ions. The effect of thisis that the electrons acquire a temperature that is typically on theorder of 1-5 ev even though the other particles in the plasma remainsubstantially at the temperature of the walls of the plasma reactorchamber (on the order of 0.03 ev) or up to a few hundred degreesCentigrade hotter.

Because the electrons are much more mobile than the ions, they initiallystrike the walls of the reactor chamber at a greater rate than do theions. The effect of this is that the plasma body becomes slightlyelectron deficient while the boundary layer sheath becomes substantiallyelectron deficient. This also produces a positively charged layer at theinterface between the plasma body and the plasma sheath. This netpositive charge of the plasma body and boundary layer results in aplasma body electrical potential V_(p) (usually called the "plasmapotential") on the order of several times the electron mean kineticenergy divided by the electron charge. The potential in the bulk of theplasma is nearly constant while the largest part of the potentialvariation is across the sheath. in an rf plasma, this sheath potentialvariation is also dependent on various parameters including the area ofthe reactor chamber wall, the area of the powered electrode, thepressure in the reactor chamber and the rf power input.

In accordance with the illustrated preferred embodiment, a plasmaapparatus is presented that operates at frequencies higher than the13.56 MHz frequency utilized in conventional plasma apparatus. Usefulfrequencies for plasma processing have been found to range from 30 to200 MHz. These processes include wafer cleaning, chemical vapordeposition and plasma etching. The frequency that is selected isdependent on which of these processes is being utilized which in turndetermines the required choice of plasma density and ion bombardmentenergy.

In the case of a nonreactive etch process, the lower frequency limit iscontrolled by the maximum self bias voltage that can be used withoutdamaging the integrated circuit under fabrication. The upper frequencylimit is controlled by the minimum self bias voltage (about 150 voltsfor a nonreactive etch process and 50 volts for a reactive ion etchprocess) that produces sufficient energy to etch the wafer. As apractical matter, when this etch process is used in a serial waferfabrication system, this upper limit is further reduced by therequirement that such etching be achievable within a period that isshort enough that this etching step does not create a bottleneck tofabrication throughput. This range of frequencies has requiredmodification of the match network that prevents reflections of rf powerat the transition from the 50 ohm characteristic impedance of the rftransmission line to the much lower impedance of the plasma reactorchamber. The plasma is generated by detuning the match network to makethe powered electrode function as an igniter and then is tuned to reducethe self-bias voltage to a level appropriate for etching wafers withoutdamage to the wafers.

In the case of plasma wafer cleaning, the frequency is chosen to producea high current density at voltages that do not etch the wafer or implantions into the wafer. In the case of plasma enhanced chemical vapordeposition, the bombardment voltage and current should be compatiblewith good deposition uniformity, high film purity and the appropriatelevel of film stress.

DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the structure of a typical plasma reactor.

FIG. 2 illustrates a match network suitable for coupling rf power atfrequencies significantly above 13.56 MHz to a plasma reactor.

FIG. 3 is a flow diagram of a process utilizing the plasma reactor andmatch network of FIGS. 1 and 2, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The fundamental nonreactive etch prior to metal 2 deposition typicallyis executed at an etch rate on the order of 250 Angstroms per minute(equivalent silicon dioxide etch rate) in order to complete this etchstep within about 40 seconds. At this rate, this etch step duration plussystem overhead to transfer wafers into and out of the reactor meets asystem throughput requirement of 60 wafers per hour. Many IC circuitdesigns today contain layers that can be damaged by bombardment by highenergy ions. To avoid such damage while retaining an etch time on theorder of 40 seconds, the plasma is generated by an rf field of frequencyhigher than the 13.56 ISM (industry, scientific, medical) frequency thatis conventionally utilized.

Because it is important to maintain system throughput, as the frequencywas varied in these tests, the power was adjusted to achieve a 250Angstroms per minute equivalent silicon dioxide etch rate For such etchrate, the negative self bias voltage was measured to be just over 500volts for a 40 MHz frequency and was measured to be approximately 310volts for a 60 MHz frequency. This confirms that soft etches (i.e.,etches with self bias on the order of or less than 500 volts) withacceptable throughput can be produced at these frequencies. Theserf-bias voltage for comparable powers at 137 MHz is -125 volts.Therefore, it appears that a useful range of frequencies for achieving asoft etch that etches the wafer without damaging thin layers extendsfrom 30 MHz to 200 MHz. Frequencies above 137 MHz are particularlyuseful for wafer cleaning and plasma enhanced chemical vapor deposition.The preferred choice of frequency is in the range 50-70 MHz because thisproduces the required etch rate under very soft conditions (self-biasnear -300 V). For a frequency f in the range 13.56 MHz<f<200 MHz and anrf power that produces a 250 Angstroms per minute equivalent silicondioxide etch rate, the etch uniformity is comparable to the uniformityexhibited by conventional 13.56 MHz nonmagnetized processes. For anonreactive etch, the gas pressure is selected in the range from 1 to 20milliTorr.

A soft etch process that uses high frequency rf power is useful in bothnonreactive, nonselective etch processes and in reactive ion etchprocesses. The power is generally higher and the pressure is generallylower for the nonreactive ion processes. For example, for an etch withargon ions, the power should be on the order of 300 Watts and thepressure should be on the order of 4 mTorr. A reactive ion etch normallyemploys fluorine-containing or chlorine-containing gases. For example,the reactive ion etch containing 10 mole percent NF₃ or 5 mole percentBCl₃, the power should be on the order of 10-50 Watts and the pressureshould be on the order of 10-40 mTorr.

For a cleaning process, the pressure is typically selected in the rangefrom 1 to 40 milliTorr and the self-bias voltage is selected in therange from 5 to 300 volts. Preferably, the pressure is on the order of 5milliTorr and the self-bias voltage is on the order of 15 volts. Toachieve these parameter values, the frequency should be selected in therange from 100 to 200 MHz. Particularly useful gases for wafer cleaningare pure argon, hydrogen and gas mixtures that include afluorine-containing gas.

The frequency can also be selected to optimize various plasma enhancedchemical vapor deposition processes. For example, for that of silicondioxide, the total process pressure can range from 0.5 to 20 milliTorr.The optimum pressure is on the order of 5 milliTorr. The self-biasvoltage is typically in the range from 10 to 400 volts and ispreferrably selected to be on the order of 150 volts. To achieve theseparameter values, the frequency should be selected in the range from 100to 200 MHz. Particularly useful gases for plasma enhanced chemical vapordeposition are argon, silane and TEOS.

A pair of electromagnetic coils 114 and 115 and associated power supply116 are included to produce a weak magnetic field that deflects plasmaions away from the walls of the plasma reactor. This is important toavoid contamination of the wafer during processing. Unlike in the priorart, it is not necessary that these fields be uniform at the surface ofthe wafer because they are too weak (on the order of 1-20 Gauss at thesurface of the wafer) to significantly affect process uniformity.However, this range of magnetic field is sufficient to prevent plasmaions from impacting the walls with sufficient energy to desorbcontaminants from those walls.

A match network is used to couple power from the 50 ohm impedance rfpower line to the much lower impedance plasma without producing asignificant amount of reflected power at the match network. For thefrequencies significantly above 13.56 MHz (i.e., on the order of orgreater than 40 MHz), the conventional match network design cannot beused.

At rf frequencies, the wavelength of the signals becomes small enoughthat phase variation of signals over the lengths of cables can producesignificant interference effects. In this frequency range, cables shouldbe substantially equal to an integral number of quarter wavelengths. Inparticular, cable 111 from rf power supply 15 to match network 112 andcable 113 from match network 112 to powered electrode 16 should each besubstantially equal to an integral number of quarter wavelengths. By"substantially equal to" is meant that this length is equal to anintegral number of quarter wavelengths plus or minus 0.05 quarterwavelength. This requirement is easily met at 13.56 MHz where a quarterwavelength is on the order of 15 feet, so a small cutting error of thelength of such cable will not be significant. However, at 60 MHz, aquarter wavelength is about 3 feet so that cable length errors areproportionately 5 times as significant. The addition of a single extraconnector or circuit element can violate this cable length criterion.

In addition, at these frequencies, the discrete components, such asinterdigitated blade capacitors and multiple coil inductors,conventionally used in the match network for a 13.56 MHz plasmaapparatus are unsuitable for use at the higher frequencies. Theinductances of such discrete components are too large for frequencies onthe order of or greater than 40 MHz. In the chosen range of frequencies,an inductor can be a single strip of conductor. Likewise, the multibladeinterdigitated blade capacitors of 13.56 MHz systems are replaced by asimple pair of parallel conductive plates spaced by a nonconductor suchas a teflon sheet.

FIG. 2 illustrates a match network design that can be used atfrequencies above 40 MHz. A ground 20 is connected to the outerconductor of a first rf connector 21 located in a wall 22 of matchnetwork 112. A conductor 23 electrically connects the inner conductor ofconnector 21 to a first plate 24 of an input capacitor 27. Thiscapacitor also contains a second plate 26 and a dielectric spacer 25.Plate 26 is electrically connected to a first plate 28 of a shuntcapacitor 211 that also contains a dielectric spacer 29 and a secondplate 210. Plate 28 is also connected to a first plate 212 of a thirdcapacitor 215, that also includes a dielectric spacer 213 and a secondplate 214. Plate 214 connects through an inductor 216 to rf electrode16.

To permit tuning of this match network at a given frequency in the range40-100 MHz, capacitors 27 and 211 are variable capacitors. In thisembodiment, these capacitances are varied by variation of the spacingbetween plates 24 and 26 and between the spacing between plates 28 and210. Variation of these spacings is achieved by means of a motor 221connected by a rotary-to-linear-displacement coupling 222 and a motor223 connected by a rotary-to-linear-displacement coupling 224. Anautomated control circuit 225 automatically adjusts these twocapacitances to minimize the amount of power reflected at rf coupler 21.To enable such adjustment, a detector 226, connected between rf powersupply 15 and rf connector 21 provides to control circuit 225information about the relative phase between the current and voltagecomponents of the rf power input; and the ratio of the magnitudes of thecurrent and voltage components of the rf power input signal. Controlcircuit 225 is a conventional feedback control circuit that adjusts theplate spacings of capacitors 27 and 211 until the relative phase andratio of magnitudes of the rf current and voltage signals reaches presetvalues that are selected to produce substantially zero reflection ofpower back toward the rf power source. Typically, for a tuned matchnetwork, this system will produce less than 10 Watts of reflected powerfrom a 300 Watt input signal.

For operation over the range of rf frequencies from 40-100 MHz,components 27, 211, 215 and 216 should have the values 10-100 pf, 50-400pf, 100 pf and 0.5 μH, respectively.

The variable control of the capacitors enables electrode 16 to functionas an igniter electrode to generate a plasma as well as the poweredelectrode to maintain the plasma. When it is utilized as the igniterelectrode, the feedback control of capacitors 27 and 211 is inactivatedand these capacitances are set respectively to 100 pf and 400 pf. Thisproduces at electrode 16 an electric field strength which issufficiently large to produce a cascade of electrons that ignites theplasma. After ignition and tuning, the self-bias voltage on the poweredelectrode is on the order of -300 volts for 300 Watts of power at 60MHz.

Because this plasma apparatus can be operated over a range offrequencies, the chosen frequency will not in general be one of the ISM(industrial, scientific, medical) frequencies. Therefore, rf gasketingis used in all vacuum flanges of reactor 10, no windows into chamber 12are allowed, and all elongated openings in the chamber are eliminated orshielded so that rf radiation from reactor 10 is eliminated to an extentthat reduces rf emissions below the United States governmentally allowedlevel of 15μV/m at a distance of 300 m from the apparatus. This avoidsinterference with TV and other rf communication near such reactors.Although tests have indicated that a faster etch without significantdamage to the wafer is achieved at 60 MHz than at 40 MHz, the frequencyof 40.68 MHz is an attractive choice because it is an ISM standardfrequency with reduced radiation limits. Harmonics of this frequency arestill a problem, but the power in these harmonics is generallysubstantially less than at the fundamental.

We claim:
 1. A process for performing a nonreactive plasma soft etchcomprising the steps of:(a) providing an inert gas mixture within aplasma reactor chamber; and (b) coupling RF power to an electrode withinthe chamber, the RF power being of a frequency substantially higher than13.56 MHz; (c) wherein the RF power level and frequency are selected soas to excite the gas mixture to a plasma state and so as to produce aself-bias on said electrode less than or equal to 500 volts.
 2. Theprocess of claim 1 wherein said frequency is in a range of 30 to 200MHz.
 3. The process of claim 2 wherein said frequency is above 137 MHz.4. The process of claim 1 wherein said frequency is above 137 MHz. 5.The process of claim 1 wherein a pressure of said inert gas mixturewithin said plasma reactor chamber is in a range from 1 to 20 milliTorr.6. The process of claim 1 wherein said inert gas mixture comprisesargon.
 7. The process of claim 6 wherein said inert gas mixture consistsessentially of argon.